Manganese dioxide coatings on both internal and external surfaces of anodized electrolytic capacitor anodes serve as the cathode for most commercially available "solid" capacitors. Typically, powdered tantalum or other suitable valve metal is pressed and sintered to form a porous anode body which serves as the anode. Other suitable valve metals include titanium, niobium, zirconium, aluminum, hafnium, tungsten, or mixtures, alloys, or metallic-glass compositions of these metals. A capacitor dielectric is then formed by electrolytic oxidation of the surfaces of the porous anode body. A semiconductor manganese dioxide coating is then deposited upon these surfaces to serve as the cathode in the electrolytic capacitor construction.
The manganese dioxide coating is typically produced by dipping the anodized anodes in an aqueous manganous nitrate solution followed by pyrolytic decomposition of the manganous nitrate to manganese dioxide in an oven, usually at a temperature of 130-270.degree. C., for sufficient time to substantially complete the pyrolytic conversion of the manganous nitrate. This process is repeated several times using various manganous nitrate concentrations to form a semi-conducting film providing adequate coverage of all anode surfaces.
Modern electronics circuits employing capacitors function most efficiently with capacitors having relatively low equivalent series resistance (ESR) and dissipation factor (df). In order to produce electrolytic capacitors containing manganese dioxide cathodes and having relatively low ESR and df, it is important that the capacitors contain highly conductive manganese dioxide.
Manganese dioxide, both naturally occurring and synthetic, is a complex substance which occurs in a variety of crystal forms, densities, hydration states, and metal/oxygen stoichiometries. These differences impact the electrical conductivity of manganese dioxide to a marked degree. Generally speaking, the most conductive material appears to be the beta crystal form, which also has the highest density and lowest water content, and is closest to true MnO.sub.2 stoichiometry.
Capacitor manufacturers have attempted to maximize the electrical conductivity of pyrolytic manganese dioxide by optimizing the manganous nitrate specific gravity dip sequence, oven temperature, and the composition of the oven atmosphere. A process for injecting steam into a conversion oven was described in U.S. Pat. No. 3,337,429. U.S. Pat. No. 5,622,746 describes a process for producing high conductivity pyrolytic manganese dioxide coatings by injecting highly oxidizing species, such as nitric acid or ozone, into the pyrolysis oven. However with nitric acid injection, control of the atmosphere is difficult, pyrolysis oven redesign is necessary to limit the volumes of nitric acid required, and the acidic atmosphere tends to cause the manganous nitrate to splatter, depositing MnO.sub.2 on the capacitor positive lead. It is also desirable from an environmental standpoint to avoid the generation of excess NO.sub.2 gases which result from the decomposition of the additional nitric acid injected into the pyrolysis oven.
U.S. Pat. Nos. 4,038,159 and 4,042,420 disclose highly conductive manganese dioxide coatings on tantalum capacitors produced through the thermal decomposition of aqueous manganous nitrate solutions in a small, positive pressure, radiant energy oven. However, there is still the problem of manganous nitrate splatter with small, positive pressure radiant furnaces. See, for example, U.S. Pat. No. 4,038,159, column 5, lines 44-52.
U.S. Pat. No. 3,801,479 describes a method for incorporating electrolytic manganese dioxide into a tantalum capacitor anode followed by a heat treatment or high temperature anodizing step to improve the leakage current of capacitors via restoration of stoichiometry of the tantalum oxide dielectric with oxygen from water released by the electrolytic manganese dioxide (FIG. 1 and equation 7, line 14, column 5).
The literature of the tantalum capacitor industry generally describes the product of the pyrolytic decomposition of manganous nitrate as beta-MnO.sub.2. Wiley, et. al. describes the pyrolytic conversion of manganous nitrate in test tubes to form manganese dioxide in the paper entitled "The Electrical Resistivity of Pyrolytic Beta MnO.sub.2," Journal of the Electrochemical Society, Vol. 111, June 1964. The authors indicate that x-ray diffraction patterns of the samples produced were consistent with beta-MnO.sub.2. The resistivities of the samples produced were essentially constant for conversion temperatures between 150 and 370.degree. C.
In a paper entitled "Electrical Properties of Manganese Dioxide and Manganese Sesquioxide," Journal of the Electrochemical Society, Vol. 117, No. 7, July 1970, Peter Klose describes manganese oxides produced by the pyrolytic decomposition of manganous nitrate in a variety of vessels. While Klose did not perform x-ray diffraction analysis of his samples, others in the literature routinely refer to them as beta-MnO.sub.2 (e.g. E. Preisler, "Semiconductor Properties of Manganese Dioxide," Journal of Applied Electrochemistry, 6,311 (1976) and Jian-Bao Li, et. al., "Electrical Properties of Beta and Gamma Type Manganese (IV) Oxides," Journal of the Ceramic Society of Japan, 96, 74 (1988)). U.S. Pat. No. 3,801,479 describes manganese dioxide used in solid tantalum capacitors as beta-MnO.sub.2.
In a paper entitled "Electrical and Physical Properties of MnO.sub.2 Layer for the High Performance Tantalum Solid Electrolytic Capacitor", presented at the 2nd Manganese Dioxide Symposium in Tokyo in 1980, the researchers claim that the pyrolytic conversion of manganous nitrate in both forced convection and radiation furnaces result in beta-MnO.sub.2 over a wide range of conversion temperatures (200 to 300.degree. C.).
U.S. Pat. No. 3,801,479 describes the pyrolytic MnO.sub.2 on tantalum capacitors as beta-MnO.sub.2 (in FIG. 1 and in lines 46-49, column 3). This patent discloses the departure from stoichiometry produced in the anodic tantalum oxide by the pyrolysis process and exploits the resulting large dependence of the anodic oxide resistivity upon temperature to produce a uniform layer of electrolytic manganese dioxide on the pyrolized capacitor bodies from solutions containing manganese ions and maintained at a temperature between 50.degree. C. and 99.degree. C. (claim 7). The uniform currents required for the production of the electrolytic manganese dioxide layer are not obtained unless the anodic tantalum oxide stoichiometry is first disturbed by exposure to pyrolysis temperatures (lines 8-12, column 4). The leakage current of capacitors produced by the process described in U.S. Pat. No. 3,801,479 is reduced to produce high-quality capacitors by restoration of the anodic tantalum oxide stoichiometry via a heat treatment step during which the tantalum anode is externally biased neutral or positive (lines 31-33, column 5) to produce migration of oxygen to the tantalum oxide surface (equation 7, line 15, column 5). The importance of the application of voltage at temperatures above 110.degree. C. is stressed in lines 62-71, column 7.
While beta-MnO.sub.2 is formed by the pyrolytic decomposition of manganous nitrate in a test tube, unless special measures are taken to control the oven atmosphere, the primary product of the decomposition of manganous nitrate on tantalum capacitors is a form of MnO.sub.2 referred to as gamma-MnO.sub.2 or epsilon-MnO.sub.2 (ahktenskite). X-ray diffraction studies of pyrolytic manganese dioxide sample produced under a wide range of pyrolysis temperatures (130-330.degree. C.) in various atmospheres (dry to 75% steam) indicate the less conductive forms of MnO.sub.2 are the principle reaction product. Beta-MnO.sub.2 is produced by the decomposition of manganous nitrate if nitric acid is injected into the oven atmosphere (U.S. Pat. No. 5,622,746). This process however is difficult to control, and it is desirable to avoid the introduction of concentrated nitric acid into a production environment.
E. Preisler ("Semiconductor Properties of Manganese Oxide," Journal of Applied Electrochemistry, 6,311 (1976) describes a heat treatment which transforms the properties of elecrodeposited manganese oxides from gamma-MnO.sub.2 to beta-MnO.sub.2. R. Giovanoli ("A Review of Structural Data of Electrolytical and Chemical MnO.sub.2," 2nd MnO.sub.2 Symposium in Tokyo (1980) reported that lattice transformations could be observed in gamma-MnO.sub.2 chemically prepared from Na.sub.4 Mn.sub.14 O.sub.27 *9 H.sub.2 O by digestion in dilute nitric acid (CMD). Evidence of the lattice transformations were also observed in gamma-MnO.sub.2 prepared by electrolytic methods (EMD).
Jian-Bao Li, et al. ("Electrical Properties of Beta- and Gamma-Type Manganese (IV) Oxides," Journal of the Ceramic Society of Japan, 96, 74 (1988) describe the effect of various heat treatments on beta-MnO.sub.2 and gamma-MnO.sub.2. Gamma-MnO.sub.2 samples exhibited minimum resistivity following heat treatment in the temperature range 350-400.degree. C. Beta-MnO.sub.2 exhibited small increases in resistivity following exposures to temperatures in excess of 150.degree. C. Peter Klose ("Electrical Properties of Manganese Dioxide and Manganese Sesquioxide," Journal Of The Electrochemical Society, 111,656 (1960) also reports an increase in the resistivity of pyrolytic MnO.sub.2 following exposure to temperatures in excess of 200.degree. C. P. Fau, et al. ("Electrical Properties of Sputtered MnO.sub.2 Thin Films," Applied Surface Science, 78, 203 (1994) describes a decrease in the resistivity of sputtered thin films of MnO.sub.2 by air annealing at temperatures up to 450.degree. C.